Update dependabot.yml

[ashleysommer]

Use the new "increase-if-necessary" strategy for Dependabot to reduce the occurrence of content-hash conflicts.

Summary of changes

Checklist

• [ ] Checked that there aren't other open pull requests for the same change.
• [ ] Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • [ ] Created an issue to discuss the change and get in-principle agreement.
  • [ ] Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • [ ] Added or updated tests that fail without the change.
  • [ ] Updated relevant documentation to avoid inaccuracies.
  • [ ] Considered adding additional documentation.
• [ ] Considered granting push permissions to the PR branch, so maintainers can fix minor issues and keep your PR up to date.

[ashleysommer]

Another update. I'll set it to "widen", because the semantics for "widen" are already the same as "if-necessary", it is actually like "widen-if-necessary".